In the production of printed circuit boards which are manufactured by wave soldering techniques, it is known to apply liquid flux to the underside of the circuit boards by foam or wave fluxing techniques. These methods of application have liquid open to the atmosphere which allows the liquid to degenerate, either by evaporation of the solvent component of the flux or by absorption of airborne water and contamination. Moreover, such fluxes are highly flammable and are hazardous to the health of machine operators.
Both moral and legislative environmental considerations have prompted the development of new application techniques. This has led to the introduction of flux formulations having low solids content and no cleaning in order to eliminate the necessity for flux removal by CFC or aqueous based cleaning after the soldering process. Unfortunately, these fluxes degrade easily and it has been difficult to control their composition by automatic means.
Various methods have been proposed to eliminate open liquid vessels which lead to flux degradation and present a health and fire hazard but the very nature of the fluxing medium has led to persistent blockage and an unacceptable degree of maintenance. Moreover, in the known systems, deposition rates have been variable, notably with respect to conveyor speeds and the usually air generated high energy sprays employed have resulted in unwanted deposits on the top side of the circuit boards and overspray has been difficult to contain even if collected adequately by local extraction thereby leading to environmental pollution by exhaust to atmosphere.